The present invention relates generally to a method and apparatus for using pulse echo techniques to detect fluid flow behind an acoustically reflective structure. More particularly, it relates to a method for detecting fluid flow behind casing in a borehole.
The use of pulse echo techniques in the borehole environment is not new. The use of a variety of devices employing such techniques for the evaluation of formations that are penetrated by a borehole in the search for hydrocarbon deposits is well known. Many such devices have been referred to generically as "borehole televiewers" (BHTVs). The leading reference in this field of science and technology is U.S. Pat. No. 3,369,626 entitled "METHOD AND APPARATUS FOR PRODUCING A VISUAL RECORD OF PHYSICAL CONDITIONS OF MATERIALS TRAVERSED BY A BOREHOLE," issued on Feb. 20, 1968 to J. Zemanek, Jr. Since the issuance of this patent, numerous patents and technical papers pertaining to this field have been published. Accordingly, a further description of the art is deemed unnecessary at this time.
Although pulse echo techniques which involve measurement of the Doppler effect have been used for years to measure flow velocities, they are effective only when the flow being measured has a velocity component which is parallel to the acoustic beam transmitted by the device in use; they are ineffective when the transmitted acoustic beam is perpendicular to the flow velocity. Further, such devices have been ineffective in measuring flow behind an acoustically reflective barrier, e.g., casing in a borehole, essentially because of the magnitude of the acoustic return from the reflective barrier.
In the exploration and production of hydrocarbons, boreholes are normally "cased" prior to completion of a well by installing a tubular casing in the borehole which is cemented into place by cement which is disposed between the casing and the wall of the borehole. It is desirable to be able to detect and measure fluid flow behind the casing, i.e., between the casing and the wall of the borehole. It will be noted that, as used herein, "fluid flow" refers to the flow of liquids, gases, or a combination of both. Obviously, it is advantageous to be able to accomplish this objective with a method that includes the use of a tool that can be deployed through the central bore of the casing.